Interest in catalysis continues to grow in the polyolefin industry. Many olefin polymerization catalysts are known, including conventional Ziegler-Natta catalysts. To improve polymer properties, single-site catalysts, in particular metallocenes, are beginning to replace Ziegler-Natta catalysts. Single-site catalysts typically require large amounts of expensive activators such as methylalumoxane or salts of non-nucleophilic anions such as triphenylcarbenium tetrakis(pentafluorophenyl)borate. Many single-site catalysts are difficult to synthesize. This adds to the cost of the catalyst system. It would be desirable to incorporate some of the advantages of single-site catalysts, such as narrow molecular weight distribution and good comonomer incorporation, without the high cost.
Single-site catalysts typically feature at least one polymerization-stable, anionic ligand that is purely aromatic, as in a cyclopentadienyl system. All five carbons in the planar cyclopentadienyl ring participate in η-5 bonding to the metal. The cyclopentadienyl anion functions as a 6π-electron donor. Similar bonding apparently occurs with heteroatomic ligands such as boratabenzenyl or azaborolinyl.
U.S. Pat. Nos. 5,459,116, 5,798,424, and 6,114,276 teach olefin polymerization catalysts that do not require the use of expensive activators. U.S. Pat. No. 5,459,116 uses titanium compounds reacted with hydroxyesters such as 2-hydroxyethyl methacrylate. U.S. Pat. No. 6,114,276 reacts titanium compounds with carbodiimide ligands and U.S. Pat. No. 5,798,424 prepares five-membered chelated titanium compounds. Similarly, Eur. Pat. No. 1,238,989 and U.S. Pat. No. 6,897,176 do not require the use of expensive activators and utilize five- or six-membered chelated compounds.
Other transition metal complexes containing chelating ligands are known. U.S. Pat. No. 5,637,660 describes transition metal complexes containing chelating ligands based upon pyridine or quinoline. Science and Technology in Catalysis (2002) 517 describes transition metal complexes based upon phenoxyimines supported on magnesium chloride. None of these are three-membered titanacycles.
Single-site transition metal complexes based upon amine derivatives such as alkoxyamines are known for olefin polymerizations (see, for example, U.S. Pat. Nos. 6,204,216 and 6,281,308, Organometallics 23 (2004) 836, Organometallics 23 (2004) 1405, and Chem. Commun. 16 (2005) 2152). The nitrogen atom is bonded to a heteroatom and has two additional substitutents. Although the lone pair of electrons on the nitrogen atom coordinates with the transition metal, nitrogen is not formally bonded to the metal and a 3-membered titanacycle is not used. In the example given, the Group 4 element, zirconium, is bonded to four atoms including the oxygen atom from the monoanionic ligand (prepared from n-butyllithium and N,N-diethylhydroxylamine). The nitrogen atom is not one of the four atoms formally bonded to the zirconium.
Three-membered titanacycles have been made by reaction of Ti(II) species with pi bonds (see, for example, J. Organometal. Chem. 624 (2001) 229, Organometallics 22 (2003) 24, and Eur. J. Org. Chem. (2003) 4721). For example, titanium diisopropoxide can be reacted with ketones or nitrites to give the corresponding three-membered titanacycles.
While three-membered titanacycles are known, apparently olefin polymerization processes using catalyst systems incorporating a three-membered titanacycle have not been contemplated. Such a polymerization process has many advantages in ease of preparation of a wide variety of catalyst systems.